The categories.html template is used in three places:
1) for the tabs on the result page
2) for the default category selection in the preferences
3) for the advanced search category selection in the oscar theme
The search_on_category_select plugin only makes the checkboxes in 1)
behave like tabs. The previous commit however hides the checkboxes
in all three places when the plugin is loaded, which we don't want:
The categories in 2) and 3) always act like checkboxes, so the
checkboxes in these places should actually always be visible.
Note that we inject the CSS via JavaScript instead of using
css_dependencies because we do not wan the CSS to load
when JavaScript is disabled (since in that case the checkboxes
should be visible).
